Method for processing video data for a display device

ABSTRACT

The invention is related to a new kind of pre-processing for plasma display panel control. The plasma display technology has the drawback of a reduced grey scale portrayal. This is due to the fact that contrarily to CRTs where luminance is approximately quadratic to the applied cathode voltage, luminance is linear to the number of discharge pulses in PDPs. Therefore, an approximately quadratic degamma function has to be applied to the input video signal components R,G,B before sub-field coding can be done. Truncation to 8-bit video data is required, so that the effect of the degamma function cannot be fully maintained. Especially in the region of small video levels, where the eye sensitivity is high, the grey scale portrayal is poor. 
     According to the invention it is proposed to use a new kind of dithering, adapted to the PDP specialities to improve the grey scale portrayal. These adaptation includes three dithering specialities which can be used singly or in combination. These are:
         cell-based dithering, i.e. to each color component R, G, B of a pixel separate dithering numbers are added;   object/region-based dithering, i.e. the set of disposable dithering numbers is made dependent on the region/object in the video picture;
 
the set of disposable dithering numbers is made dependent on the video (signal) level.

This application claims the benefit under 35 U.S.C. § 365 of International Application PCT/EP01/02668, filed Mar. 9, 2001, which claims the benefit of European Patent Application No. 00250099.9, filed Mar. 22, 2000.

The invention relates to a method and apparatus for processing video picture data for display on a display device. More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on matrix displays like plasma display panels (PDP) or other display devices where the pixel values control the generation of a corresponding number of small lighting pulses on the display.

BACKGROUND

The Plasma technology now makes it possible to achieve flat colour panel of large size (out of the CRT limitations) and with very limited depth without any viewing angle constraints.

Referring to the last generation of European TV, a lot of work has been made to improve its picture quality. Consequently, a new technology like the Plasma one has to provide a picture quality as good or better than standard TV technology. On one hand, the Plasma technology gives the possibility of “unlimited” screen size, of attractive thickness . . . but on the other hand, it generates new kinds of artefacts which could degrade the picture quality.

Most of these artefacts are different as for CRT TV pictures and that makes them more visible since people are used to see the old TV artefacts unconsciously.

A Plasma Display Panel (PDP) utilizes a matrix array of discharge cells which could only be “ON” or “OFF”. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, a PDP controls the grey level by modulating the number of light pulses per frame (sustain pulses). This time-modulation will be integrated by the eye over a period corresponding to the eye time response.

Since the video amplitude determines the number of light pulses, occurring at a given frequency, more amplitude means more light pulses and thus more “ON” time. For this reason, this kind of modulation is also known as PWM, pulse width modulation.

This PWM is responsible for one of the PDP image quality problems: the poor grey scale portrayal quality, especially in the darker regions of the picture. This is due to the fact, that the displayed luminance is linear to the number of pulses, but the eye response and its sensitivity to noise is not linear. In darker areas the eye is more sensitive than in brighter areas. This means that even though modern PDPs can display e.g. 255 discrete video levels for each colour component R,G,B, the quantisation error will be quite noticeable in the darker areas. Further on, the required degamma function in PDP displays, increases quantisation noise in video dark areas, resulting in a perceptible lack of resolution.

There are known some solutions which use a dithering method for reducing the perceptibility of quantisation noise. These solutions are however not oriented to the nature of the display and of the displayed video. Proposed dithering methods in the literature were mainly developed to improve quality of non-moving black and white images (fax application and newspaper photo portrayal). The obtained results are therefore not optimal if the same dithering algorithms are directly applied to PDPs.

INVENTION

To overcome the drawback of reduced grey scale portrayal, the present invention, reports a dithering technique adapted to the specific problems in PDPs.

To achieve a better grey scale portrayal, a dithering signal is added to the video signal, before truncation to the final video grey scale amplitude bit resolution. As mentioned before, dithering per se is a well-known technique from the technical literature, used to reduce the effects of quantisation noise due to a reduced number of displayed resolution bits. With dithering, some artificial levels are added in-between the existing video levels. This improves the grey scale portrayal, but on the other hand adds high frequency, low amplitude dithering noise which is perceptible to the human viewer only at a small viewing distance.

The solution according to the invention makes an adaptation of the dithering signal to the PDP specialities in order to achieve an optimised grey-scale portrayal and a minimised dithering noise at the same time. There are three concrete techniques which can be used singly or in combination for the optimisation. These are:

-   -   Cell-based dithering: adaptation to the cell structure of the         plasma display.     -   Object-based dithering: adaptation to the structure of the         displayed video picture.     -   Amplitude-based dithering: adaptation to the amplitude level of         the pixels or pixel regions in the displayed video picture.

Cell-based dithering consists in adding a dithering signal that is defined for every plasma cell (there are 3 plasma cells R,G,B for each pixel) and not for every pixel. This makes the dithering noise finer and less noticeable to the human viewer.

Object-based dithering means enabling addition of a dithering signal only for certain picture content objects, or to adapt the set of disposable dithering numbers to the bit resolution of the displayed objects. In other words, the bit resolution for the dithering numbers is made adaptive to the bit resolution of the displayed objects. Two examples will help to clarify this idea:

-   1. OSD (On-Screen Display) is mostly generated with 4-bits of     resolution per colour component R,G,B. This means that the display     grey scale resolution (8 bit for each colour component R,G,B) is     more than enough to correctly portray this kind of OSD, and     therefore adding a dithering signal would only add dithering noise,     without bringing a noticeable benefit. -   2. If a PC graphic card is connected to the plasma display, for     instance in 256-color mode, it is also useless to add a dithering     signal. The bit resolution for each colour component R,G,B is also     very low in this mode. Use of a dithering technique would not     improve the grey scale portrayal. It is likely that the graphics     card would add in series an own dithering signal to compensate for     the reduced number of colours.

Amplitude-based dithering means that the set of disposable dithering numbers is made a function of the amplitude of the video signal components. Also here, in other words, this could be expressed that the bit resolution for the dithering numbers is made adaptive to the video signal component amplitude. Contrary to the smaller (darker) video values, large values of video do not loose bit resolution with the application of the quadratic degamma function. Therefore, the number of dithering bits can be reduced as a function of the amplitude.

Further advantageous embodiments are apparent from the dependent claims.

DRAWINGS

Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

FIG. 1 shows an illustration for the plasma cell activation with small pulses in sub-fields;

FIG. 2 shows an illustration for pixel-based and cell-based dithering;

FIG. 3 shows an illustration of a 3-dimensional cell-based dithering pattern;

FIG. 4 shows a block diagram of a circuit implementation of the invention in a PDP.

EXEMPLARY EMBODIMENTS

In FIG. 1, the general concept of light generation in plasma display panels is illustrated. As mentioned before, a plasma cell can only be switched on or off. Therefore, the light generation is being done in small pulses where a plasma cell is switched on. The different colours are produced by modulating the number of small pulses per frame period. To do this a frame period is subdivided in so called sub-fields SF. Each sub-field SF has assigned a specific weight which determines how many light pulse are produced in this sub-field SF. Light generation is controlled by sub-field code words. A sub-field code word is a binary number which controls sub-field activation and inactivation. Each bit being set to 1 activates the corresponding sub-field SF. Each bit being set to 0 inactivates the corresponding sub-field SF. In an activated sub-field SF the assigned number of light pulses will be generated. In an inactivated sub-field there will be no light generation. A typical sub-field organisation with 12 sub-fields SF is shown in FIG. 1. The sub-field weights are listed at the top of the figure.

The frame period is illustrated slightly longer than all the sub-field periods together. This has the reason that for non-standard video sources the video line may be subject of jittering and to make sure that all sub-fields SF fits into the jittering video line, the total amount of time for all sub-fields SF is slightly shorter than a standard video line.

For clarification, a definition of the term sub-field is given here: A sub-field is a period of time in which successively the following is being done with a cell:

-   -   1. There is a writing/addressing period in which the cell is         either brought to an excited state with a high voltage or with         lower voltage to a neutral state.     -   2. There is a sustain period in which a gas discharge is made         with short voltage pulses which lead to corresponding short         lighting pulses. Of course only the cells previously excited         will produce lighting pulses. There will not be a gas discharge         in the cells in neutral state.     -   3. There is an erasing period in which the charge of the cells         is quenched.

As mentioned before, plasma uses PWM (pulse width modulation) to generate the different shades of grey. Contrarily to CRTs where luminance is approximately quadratic to the applied cathode voltage, Luminance is linear to the number of discharge pulses in PDPs. Therefore, an approximately quadratic degamma function has to be applied to the input video signal components R,G,B before the PWM.

The effect of this degamma function on the input video data is shown in the following table, where a quadratic degamma function is applied (calculated with 16-bit resolution). After applying the quadratic degamma function to the input video data, in the next column the effect of this degamma function is depicted. The numbers in this column were achieved after dividing the quadratic numbers in the previous column by 256 and truncation. By doing this it is assured that the output video range and the input video range is identical.

11 Bit 8 Bit Input 16 Bit 8 Bit Out- 11 Bit 8 Bit Input 16 Bit De- 8 Bit Output Degamma Video Degamma put Video Degamma Video Data gamma Data Video Data Data Data Data Data Data (X) (X**2) (X**2/255) (X**2)/32 (X) (X**2) (X**2/255) (X**2)/32 0 0 0 0 128 16384 64 512 1 1 0 0 129 16641 65 520 2 4 0 0 130 16900 66 528 3 9 0 0 131 17161 67 536 4 16 0 0 132 17424 68 544 5 25 0 0 133 17689 69 552 6 36 0 1 134 17956 70 561 7 49 0 1 135 18225 71 569 8 64 0 2 136 18496 72 578 9 81 0 2 137 18769 73 586 10 100 0 3 138 19044 74 595 11 121 0 3 139 19321 75 603 12 144 0 4 140 19600 76 612 13 169 0 5 141 19881 77 621 14 196 0 6 142 20164 79 630 15 225 0 7 143 20449 80 639 16 256 1 8 144 20736 81 648 17 289 1 9 145 21025 82 657 18 324 1 10 146 21316 83 666 19 361 1 11 147 21609 84 675 20 400 1 12 148 21904 85 684 21 441 1 13 149 22201 87 693 22 484 1 15 150 22500 88 703 23 529 2 16 151 22801 89 712 24 576 2 18 152 23104 90 722 25 625 2 19 153 23409 91 731 26 676 2 21 154 23716 93 741 27 729 2 22 155 24025 94 750 28 768 3 24 156 24336 95 760 29 841 3 26 157 24649 96 770 30 900 3 28 158 24964 97 780 31 961 3 30 159 25281 99 790 32 1024 4 32 160 25600 100 800 33 1089 4 34 161 25921 101 810 34 1156 4 36 162 26244 102 820 35 1225 4 38 163 26569 104 830 36 1296 5 40 164 26896 105 840 37 1369 5 42 165 27225 106 850 38 1444 5 45 166 27556 108 861 39 1521 5 47 167 27889 109 871 40 1600 6 50 168 28224 110 882 41 1681 6 52 169 28561 112 892 42 1764 6 55 170 28900 113 903 43 1849 7 57 171 29241 114 913 44 1936 7 60 172 29584 116 924 45 2025 7 63 173 29929 117 935 46 2116 8 66 174 30276 118 946 47 2209 8 69 175 30625 120 957 48 2304 9 72 176 30976 121 968 49 2401 9 75 177 31329 122 979 50 2500 9 78 178 31684 124 990 51 2601 10 81 179 32041 125 1001 52 2704 10 84 180 32400 127 1012 53 2809 11 87 181 32761 128 1023 54 2916 11 91 182 33124 129 1035 55 3025 11 94 183 33489 131 1046 56 3136 12 98 184 33856 132 1058 57 3249 12 101 185 34225 134 1069 58 3364 13 105 186 34596 135 1081 59 3481 13 108 187 34969 137 1092 60 3600 14 112 188 35344 138 1104 61 3721 14 116 189 35721 140 1116 62 3844 15 120 190 36100 141 1128 63 3969 15 124 191 36481 143 1140 64 4096 16 128 192 36864 144 1152 65 4225 16 132 193 37249 146 1164 66 4356 17 136 194 37636 147 1176 67 4489 17 140 195 38025 149 1188 68 4624 18 144 196 38416 150 1200 69 4761 18 148 197 38809 152 1212 70 4900 19 153 198 39204 153 1225 71 5041 19 157 199 39601 155 1237 72 5184 20 162 200 40000 156 1250 73 5329 20 166 201 40401 158 1262 74 5476 21 171 202 40804 160 1275 75 5625 22 175 203 41209 161 1287 76 5776 22 180 204 41616 163 1300 77 5929 22 185 205 42025 164 1313 78 6084 23 190 206 42436 166 1326 79 6241 24 195 207 42849 168 1339 80 6400 25 200 208 43264 169 1352 81 6561 25 205 209 43681 171 1365 82 6724 26 210 210 44100 172 1378 83 6889 27 215 211 44512 174 1391 84 7056 27 220 212 44944 176 1404 85 7225 28 225 213 45369 177 1417 86 7396 29 231 214 45796 179 1431 87 7569 29 236 215 46225 181 1444 88 7744 30 242 216 46656 182 1458 89 7921 31 247 217 47089 184 1471 90 8100 31 253 218 47524 186 1485 91 8281 32 258 219 47961 188 1498 92 8464 33 264 220 48400 189 1512 93 8649 33 270 221 48841 191 1526 94 8836 34 276 222 49284 193 1540 95 9025 35 282 223 49729 195 1554 96 9216 36 288 224 50176 196 1568 97 9409 36 294 225 50625 198 1582 98 9604 37 300 226 51076 200 1596 99 9801 38 306 227 51529 202 1610 100 10000 39 312 228 51984 203 1624 101 10201 40 318 229 52441 205 1638 102 10404 40 325 230 52900 207 1653 103 10609 41 331 231 53361 209 1667 104 10816 42 338 232 53824 211 1682 105 11025 43 344 233 54289 212 1696 106 11236 44 351 234 54756 214 1711 107 11449 44 357 235 55225 216 1725 108 11664 45 364 236 55696 218 1740 109 11881 46 371 237 56169 220 1755 110 12100 47 378 238 56644 222 1770 111 12321 48 385 239 57121 224 1785 112 12544 49 392 240 57600 225 1800 113 12769 50 399 241 58081 227 1815 114 12996 50 406 242 58564 229 1830 115 13225 51 413 243 59049 231 1845 116 13456 52 420 244 59536 233 1860 117 13689 53 427 245 60025 235 1875 118 13924 54 435 246 60516 237 1891 119 14161 55 442 247 61009 239 1906 120 14400 56 450 248 61504 241 1922 121 14641 57 457 249 62001 243 1937 122 14884 58 465 250 62500 245 1953 123 15129 59 472 251 63001 247 1968 124 15376 60 480 252 63504 249 1984 125 15625 61 488 253 64009 251 2000 126 15876 62 496 254 64516 253 2016 127 16129 63 504 255 65025 255 2032

As it can be seen from the values in the columns headed 8 bit output video data, for smaller input values, many input levels are mapped to the same output level. This is due to division by 255 and truncation. In other words, for darker areas, the quantisation step is higher than for the higher areas which corresponds to non-linear quantisation. In particular the values smaller than 16 are all mapped to 0 (this corresponds to four bit video data resolution which is unacceptable for video signal processing).

Dithering is a known technique for avoiding to loose amplitude resolution bits due to truncation This technique only works if the required resolution is available before the truncation step. But this is the case in the present application, because the video data after degamma operation has 16 bit resolution and in the corresponding columns there are no two identical values. Dithering can in principle bring back as many bits as those lost by truncation. However, the dithering noise frequency decreases, and therefore becomes more noticeable, with the number of dithering bits.

1 bit-dithering corresponds to multiply the number of available output levels by 2, 2 bit-dithering corresponds to multiply the number of available output levels by 4 and 3 bit-dithering corresponds to multiply the number of available output levels by 8.

Looking at the table above, in particular to the input values less than 16 reveals that at minimum 3 bit-dithering is required to reproduce the 256 video levels more correctly with the required grey scale portrayal of a ‘CRT’ display device.

In the table above the columns headed 11 Bit Degamma Data contain the output data from the degamma unit. These values are derived from the values in the columns headed 16 Bit Degamma data by dividing them by 32 or better by truncation of 5 bits. How these values are used in the dithering process will be explained later on.

Next, the cell-based dithering will be explained in detail.

With cell-based dithering a dithering number is added to every panel cell in contrast to every panel pixel as usually done. A panel pixel is composed of three cells: red, green and blue cell. The cell-based dithering has the advantage of rendering the dithering noise finer and thus making it less noticeable to the human viewer.

Because the dithering pattern is defined cell-wise, it is not possible to use techniques like error-diffusion, in order to avoid colouring of the picture when one cell would diffuse in the contiguous cell of a different colour. This is not a big disadvantage, because it has been observed sometimes an undesirable low frequency moving interference, between the diffusion of the truncation error and a moving pattern belonging to the video signal. Error diffusion works best in case of static pictures.

Instead of using error diffusion, a static 3-dimensional dithering pattern is proposed according to this invention.

FIG. 3 shows one example for such a pattern. 3-bit-dithering is used in this example. This means that the dithering numbers have values from 0 to 7. The static 3-dimensional dithering pattern is defined for a cube of 4*4*4 cells (4-lines with 4 cells each, repeatedly taken from 4 frames). It is noted that this embodiment is only an example and that the number of dithering bits as well as the size and type of dithering pattern can be subject of modification in other embodiments of the invention.

The use of a 3 bit-dithering requires that the degamma operation is performed with 3 bits more than final resolution. The final resolution is given to be 8 bit resolution. The sub-field coding range is therefore from 0 to 255. Then the output range of the degamma operation should be from 0 to 2040. It is noted that the maximum dithering number with 3 bit dithering is 7. If this number is added to 2040, the result is 2047 which is the highest possible 11 bit binary number %11111111111. A slightly lower value than 2040. e.g. 2032 could also be used. This has the advantage that the corresponding values can simply be derived from the 16 bit degamma data by truncating the 5 least significant bits.

Some other examples: if sub-field coding range would be from 0 to 175, output range of degamma operation should be from 0 to 1400; and finally if coding range is from 0 to 127, output range should be from 0 to 1016. For every panel cell and for every frame, the corresponding dither pattern value is added to the output of the degamma function, and consequently truncated to the final number of bits.

The 3-bit dither pattern shown in FIG. 3 is static. This means that it is repeatedly used for the whole panel. From FIG. 3 it can be seen that the dither pattern is repeated in horizontal direction of the panel. However, it also repeats in vertical direction and in time direction accordingly.

It is noted that the proposed pattern, when integrated over time, always gives the same value for all panel cells. If this were not the case, under some circumstances, some cells could acquire an amplitude offset compared to other cells which would correspond to an undesirable fixed spurious static pattern.

Next, the principle of object-based dithering according to the invention is explained in greater detail. Object-based dithering corresponds to modify the number of dithering bits as a function of the displayed object. For this purpose different masking bit patterns are defined which serve as a selector for the dithering bit resolution. E.g., if the object-based dithering is used in combination with the cell-based dithering, the implementation of different dithering bit resolutions can be done as follows.

The dithering pattern as shown in FIG. 3 remains unchanged. I.e., the dithering numbers have the 3 bit resolution as before at the beginning of the dithering process. This is the highest possible bit resolution in this example. For implementing the 4 different bit resolutions 3-bit, 2-bit, 1-bit and 0-bit, 4 different masking values are defined. These are:

-   -   3-bit dithering->masko=%111=7H     -   2-bit dithering->masko=%110=6H     -   1-bit dithering->masko=%100=4H     -   0-bit dithering->masko=%000=0H

These masking bit patterns are applied to the high resolution dithering numbers by Boolean operation. This can best be explained with some examples. In the examples below the Boolean operation is the logical AND operation.

3-Bit Dithering Dithering Number Masking Bit Pattern Result %111 %111 %111 %110 %111 %110 %101 %111 %101 %100 %111 %100 %011 %111 %011 %010 %111 %010 %001 %111 %001 %000 %111 %000

2-Bit Dithering Dithering Number Masking Bit Pattern Result %111 %110 %110 %110 %110 %110 %101 %110 %100 %100 %110 %100 %011 %110 %010 %010 %110 %010 %001 %110 %000 %000 %110 %000

1-Bit Dithering Dithering Number Masking Bit Pattern Result %111 %100 %100 %110 %100 %100 %101 %100 %100 %100 %100 %100 %011 %100 %000 %010 %100 %000 %001 %100 %000 %000 %100 %000

0-Bit Dithering Dithering Number Masking Bit Pattern Result %111 %000 %000 %110 %000 %000 %101 %000 %000 %100 %000 %000 %011 %000 %000 %010 %000 %000 %001 %000 %000 %000 %000 %000

From the table for 3-bit dithering it is clear that the applied masking bit pattern has no effect on the dithering numbers. They remain unchanged and therefore, 3-bit dithering is preserved as wanted.

From the table for 2-bit dithering it is clear that the applied masking bit pattern converts the 3-bit dithering numbers into 2-bit dithering numbers. There result only 4 different output values which corresponds to 2-bit dithering as wanted.

From the table for 1-bit dithering it is clear that the applied masking bit pattern converts the 3-bit dithering numbers into 2-bit dithering numbers. There result only 2 different output values which corresponds to 1-bit dithering as wanted.

From the table for 0-bit dithering it is clear that the applied masking bit pattern converts the 3-bit dithering numbers into 0-bit dithering numbers. Every input dithering number is converted to 0 which corresponds to 0-bit dithering as wanted.

The dithering bit resolution selection with masking bit patterns has the advantage that there need not be different tables for dithering patterns and different algorithms. So that the presented solution is very efficient.

In a practical application OSD insets are coded with 0-bit dithering while the video picture is coded with 3-bit dithering. If the plasma display panel is used as a monitor for computers, window borders and icons, as well as documents might be displayed with 0-bit dithering, while wall-papers and windows with motion pictures (video scenes), e.g. AVI-files or MPG-files might have 1-bit, 2-bit or 3-bit dithering enabled.

If a video picture has been coded according to the MPEG-4 standard the object/region-based dithering can benefit from this coding. The MPEG-4 standard provides the tools for video object coding. This means that the different objects in a video scene are coded independently. In a further embodiment of the invention the number of dithering bits for the cells of an object in a picture are adapted to the kind and to the bit-resolution of the objects belonging to a given MPEG-4 sequence. For instance very often the background is darker than the rest of the picture and has low contrast. In this region the application of 3-bit dithering is therefore used. The foreground is very often brighter and mostly more rich in contrast. In this region 1 bit dithering is therefore more appropriate.

Of course, object-based dithering requires some kind of information from the video source regarding video objects. This requires a picture content analysis which can be very complicated to implement. If in a low cost application this picture content analysis implementation is considered to be too expensive, then a low cost implementation of object-based dithering can be the restriction to switching off dithering in case of On-Screen-Display insets and switching on dithering for the rest of the picture.

Next, the principle of amplitude-based dithering according to the invention is explained in greater detail. Amplitude-based dithering corresponds to modify the number of dithering bits as a function of the video component signal amplitude. This can be done in similar fashion like for the object-based dithering. There are also defined some masking bit patterns for the different amplitude ranges which are used to select a corresponding dithering bit resolution by Boolean operation with the dithering numbers.

In video technology the video signal component value range is usually from 0 to 255 (8 bit words). This range is subdivided in e.g. 4 sections. The ranges and the assigned corresponding masking bit patterns are shown below:

-   For (0<=X<32), maska=%111=7H, -   for (32<=X<64), maska=%110=6H, -   for (64<=X<128), maska=%100=4H, -   for (128<=X<=255), maska=%000=0H,     where X is the amplitude of the input video component R,G,B.

According to this embodiment of the invention in the dithering circuit section the input video signal components will be classified with respect to the amplitude range. The dithering number from the dithering pattern is taken in 3-bit resolution and the logical AND operation is performed with the corresponding masking bit pattern. The resulting value is added to the video signal component data. This is done separately for each cell. The same principle is used for object-based dithering.

Next, it is explained in greater detail how the three different dithering techniques, cell-, amplitude- and object-based dithering can be combined for an optimisation.

Taking in consideration the above mentioned example with 3-bit dithering numbers, a combined solution can be described with the following formulae: Rout=trunc[degamma[Rin]+(rdither[x,y,z] AND maska[Rin,x,y,z] AND masko[x,y,z])] Gout=trunc[degamma[Gin]+(gdither[x,y,z] AND maska[Gin,x,y,z] AND masko[x,y,z])] Bout=trunc[degamma[Bin]+(bdither[x,y,z] AND maska[Bin,x,y,z] AND masko[x,y,z])] where

-   -   Rin denotes the video level of the red input video signal         component R,     -   Gin denotes the video level of the green input video signal         component G,     -   Bin denotes the video level of the blue input video signal         component B,     -   degamma[ ] denotes the degamma function with 11 bit resolution,     -   maska[ ] denotes the amplitude-based masking value,     -   masko[ ] denotes the object-based masking value,     -   rdither[ ] denotes the cell based dithering number for the red         cells according to the used dithering pattern,     -   gdither[ ] denotes the cell based dithering number for the green         cells according to the dithering pattern,     -   bdither[ ] denotes the cell based dithering number for the blue         cells according to the dithering pattern,     -   x denotes the panel pixel number,     -   y denotes the panel line number,     -   z denotes the frame number and     -   trunc [ ] denotes truncation to 8 bit resolution, i.e.         truncation of the 3 least significant bits.

The expressions:

-   (rdither [x,y,z] AND maska [Rin,x,y,z] AND masko [x,y,z])], -   (gdither [x,y,z] AND maska [Gin,x,y,z] AND masko [x,y,z])], -   (bdither [x,y,z] AND maska [Bin,x,y,z] AND masko [x,y,z])]     therefore denote a resulting dithering number after combination with     the masking bit patterns from object- and amplitude-based dithering.

The results of this calculations is illustrated in the following tables below. The results are only shown exemplarily for three input values 8, 21, 118. This is because the full table cannot be easily displayed on paper. The effect of dithering is however obvious already from the tables below. The first table concerns the example of 3-bit dithering. It is evident that for the input value 8 due to dithering the output value is changed from 0 to 1 in two cases compared to the embodiment without dithering. For the input value 21 the output value is changed from 1 to 2 in five cases compared to the case without dithering. For the input value 118 the output value is changed from 54 to 55 in three cases. Of course, the effect of dithering is becoming smaller as the input value increases because the ratio between dithering value to input value decreases.

Maska = masko = %111 = 3-bit dithering 8 Bit Input 16 Bit De- 8 Bit De- 11 Bit De- Video gamma gamma gamma Dithering 8 Bit Output Data Data Data Data Number Video Data 8 64 0 2 7 1 6 1 5 0 4 0 3 0 2 0 1 0 0 0 21 441 1 13 7 2 6 2 5 2 4 2 3 2 2 1 1 1 0 1 118 13924 54 435 7 55 6 55 5 55 4 54 3 54 2 54 1 54 0 54

The next table lists the calculation results for 2-bit dithering. Here, the effect of dithering is of course getting smaller, as smaller dithering numbers are added. However, a difference is present only for the input value 18 where the output value is changed in only four cases and for the input value 118, where the output value is changed from 54 to 55 in only two cases.

Maska = masko = %110 = 2-bit dithering 8 Bit Input 16 Bit De- 8 Bit De- 11 Bit De- Video gamma gamma gamma Dithering 8 Bit Output Data Data Data Data Number Video Data 8 64 0 2 7 1 6 1 5 0 4 0 3 0 2 0 1 0 0 0 21 441 1 13 7 2 6 2 5 2 4 2 3 1 2 1 1 1 0 1 118 13924 54 435 7 55 6 55 5 54 4 54 3 54 2 54 1 54 0 54

The next table lists the calculation results for 1-bit dithering. Here, the effect of dithering has vanished for the input vales 8 and 118 but for the input value 21 there is still the effect that the output values have been changed from 1 to 2 in four cases. Of course there are other input values, like 12, where the effect is maintained.

Maska = masko = %100 = 1-bit dithering 8 Bit Input 16 Bit De- 8 Bit De- 11 Bit De- Video gamma gamma gamma Dithering 8 Bit Output Data Data Data Data Number Video Data 8 64 0 2 7 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 21 441 1 13 7 2 6 2 5 2 4 2 3 1 2 1 1 1 0 1 118 13924 54 435 7 54 6 54 5 54 4 54 3 54 2 54 1 54 0 54

In FIG. 4 a circuit implementation of the invention is illustrated. Input R,G,B video data is forwarded to degamma unit 10 and a dither evaluation unit 12. The degamma unit 10 performs the 11-bit degamma function and delivers 11 bit video data R,G,B at the output. The dither evaluation unit 12 computes the dithering numbers: DR for red, DG for green and DB for blue. To do that it requires the sync signals H and V to determine which pixel is currently processed and which line and frame number is valid. These information is used for addressing a lookup table in which the dithering pattern is stored. The R, G and B components are used in this unit for evaluating the amplitude masking values maska. The masking value MO, which is the object-based masking value for the current pixel, is delivered by a unit in the video source, like MPEG4 decoder. This unit is not shown. In the case that no such unit is available, the signal MO can be replaced by the fast blanking signal of an external OSD insertion circuit. Unit 12 also performs the Boolean operations according to above discussed formulae. In calculation unit 11 the resulting dithering numbers and the degamma output values are added and the 3 least significant bits of the result are truncated so that the final output values Rout, Gout and Bout are achieved. These values are forwarded to a sub-field coding unit 13 which performs sub-field coding under control of control unit 16. The sub-field code words are stored in memory unit 14. Reading and writing from and to this memory unit is also controlled by the external control unit 16. For plasma display panel addressing, the sub-field code words are read out of the memory device and all the code words for one line a collected in order to create a single very long code word which can be used for the line wise PDP addressing. This is carried out in the serial to parallel conversion unit 15. The control unit 16 generates all scan and sustain pulses for PDP control. It receives horizontal and vertical synchronising signals for reference timing.

The invention can be used in particular in PDPs. Plasma displays are currently used in consumer electronics, e.g. for TV sets, and also as a monitor for computers. However, use of the invention is also appropriate for matrix displays where the light emission is also controlled with small pulse in sub-fields, i.e. where the PWM principle is used for controlling light emission. 

1. Method for processing RGB video picture data having a video (signal) level for display on a display device having a plurality of luminous elements corresponding to the colour components of pixels of a video picture, wherein a dithering method is applied to the video data to refine a grey-scale portrayal for displaying the video pictures, wherein, within the dithering method the input video data is converted to higher bit resolution and dithering numbers are added to the input video data in higher bit resolution, followed by a step with which the input video data is converted to final bit resolution, the dithering method including at least one of: (a) the dithering is made cell-based such that to each colour component R, G, B of a given pixel, and dependent on the identity of the colour component, a different dithering number from a dither pattern is added to each colour component of a given pixel, wherein the dither pattern comprises different dithering numbers from a set of dithering numbers; (b) the bit resolution of the dithering numbers is made dependent on a region/object in the video picture; and (c) the bit resolution of the dithering numbers is made dependent on the video (signal) level.
 2. Method according to claim 1, wherein for the cell-based dithering the video picture is divided into a number of sections and a static 3-dimensional dithering pattern is defined which is used repeatedly in a video sequence, wherein a first dimension corresponds to a video line number, a second dimension corresponds to a number of cells within a video line section and a third dimension corresponds to a number of video frames.
 3. Method according to claim 2, wherein the static 3-dimensional dithering pattern is defined for a section of 4 lines with 4 cells each for a number of 4 consecutive frames with a bit-resolution for the dithering numbers of 3 bits.
 4. Method according to claim 1, wherein for the region/object-based dithering, the information about different video objects/region is taken over from an MPEG4 data stream.
 5. Method according to claim 1, wherein to each of the specific sets of dithering numbers for the dithering process, a corresponding masking bit pattern is assigned which determines by Boolean operation which of the bits of a high resolution dithering number are to be taken for the resulting final dithering number.
 6. Method according to claim 1, wherein for the video level-based dithering, a full range of the video (signal) level is subdivided in a number of sections and to each section a corresponding masking bit pattern is assigned which determines by Boolean operation which of the bits of a high resolution dithering number are to be taken for the resulting final dithering number.
 7. Method according to claim 6, wherein the full video level range from 0 to 255 is subdivided in 4 sections, in particular 0 to 31, 32 to 63, 64 to 127, and 128 to 255 and correspondingly the following bit resolutions are used for the ranges, 3-bit, 2-bit, 1-bit, 0-bit and wherein the bit resolution decreases as the video level range increases.
 8. Method according to claim 1, wherein for the combined use of all the dithering specialities the following formulae are applied: Rout=trunc[degamma[Rin]+(rdither[x,y,z] AND maska[Rin,x,y,z] AND masko[x,y,z])] Gout=trunc[degamma[Gin]+(gdither[x,y,z] AND maska[Gin,x,y,z] AND masko[x,y,z])] Bout=trunc[degamma[Bin]+(bdither[x,y,z] AND maska[Bin,x,y,z] AND masko[x,y,z])], where Rin denotes the video level of the red input video signal component R, Gin denotes the video level of the green input video signal component G, Bin denotes the video level of the blue input video signal component B, degamma[ ] denotes the degamma function with specific bit resolution, in particular 11-bit resolution, maska[ ] denotes the amplitude-based masking value, masko[ ] denotes the object-based masking value, rdither[ ] denotes the cell-based dithering number for the red cells according to the used dithering pattern, gdither[ ] denotes the cell-based dithering number for the green cells according to the used dithering pattern, bdither[ ] denotes the cell-based dithering number for the blue cells according to the used dithering pattern, x denotes the panel pixel number, y denotes the panel line number, z denotes the frame number, and trunc[ ] denotes truncation to a specific bit resolution, in particular 8-bit resolution.
 9. Method according to claim 1, wherein the method is used for the video signal processing in a Plasma display device.
 10. Apparatus for processing video pictures having a grey-scale portrayal for display on a display device having a plurality of luminous elements corresponding to the RGB colour components of pixels of a video picture, said apparatus comprising a dithering unit which calculates dithering numbers that are added to video picture data having a video (signal) level to refine the grey-scale portrayal in the video pictures, wherein, within the dithering unit conversion means that convert the input video data to higher bit resolution are comprised addition means that add dithering numbers to the input video data in higher bit resolution, and processing means performing a step with which the input video data is converted to final bit resolution, the dithering unit calculating dithering numbers according to at least one of: (a) applying different dithering numbers for the RGB colour components so that, dependent on the identity of the colour component, a different dithering number from a dither pattern is added to each of the RGB components of a given pixel; wherein the dither pattern comprises different dithering numbers from a set of dithering numbers; (b) selecting the bit resolution of dithering numbers dependent on the region/object in the video picture; and (c) selecting the bit resolution of the dithering numbers dependent on the video (signal) level. 